1. Field of the Invention
The present invention relates to a press section apparatus defining a press nip for pressing a web of paper. More particularly, the present invention relates to a press section apparatus in which the backing roll includes a granite outer surface.
2. Information Disclosure Statement
Although many attempts have been made at manufacturing artificial granite press rolls, the inherent quality of granite when fabricated as a press roll is unable to be duplicated by artificial means.
More particularly, the surface texture of a granite roll is such that after passing a formed web through a press nip defined by a press member and a rotatable granite backing roll, the resultant pressed web is easily removed from the surface of the granite backing roll. However, although granite press rolls possess ideal release characteristics, they are unable to withstand bowing along the axial length thereof because unlike their counterpart steel rolls, granite backing rolls have a very low tensile strength. Therefore, according to the present invention, a granite backing roll includes deflection compensating means so that bowing of the granite roll is inhibited.
More specifically, for certain grades of paper, the desirable release characteristics exhibited by natural granite cannot be duplicated by manufactured materials. Due to the engineering strength limitations of the granite, conventional granite roll designs are not suitable for use in high nip pressure configurations which can be up to pressures of 6,000 pounds per linear inch.
A solution to the aforementioned problem is to use a granite shell or a composite shell including a metal support and an overlying sleeve of granite so that the external applied nip load is directly opposed by an internal load on the shell or sleeve to minimize bending and circumferential stresses on the granite.
The aforementioned concept has been used for metal shelled rolls in the prior art but has not been used for rolls which have a granite shell or sleeve.
The proposed granite roll would allow the application of high nip presses such as extended nip presses to grades that require granite roll surfaces for necessary sheet release characteristics.
The roll according to the present invention includes the basic deflection compensating means of a stationary center shaft which uses one or more load shoes of hydrodynamic or hydrostatic design to apply internal pressure to the inside diameter of a natural granite shell or a metallic shell or support on which a granite sleeve has been mounted.
The internal shoes or pistons are loaded by hydraulic pressure against the inside diameter of the shell in order to oppose external loads applied to the roll.
A fluid is introduced inside the roll to provide lubrication between the stationary load shoes or pistons and the rotating internal surface of the roll.
The internal hydraulic load pressure is controlled such that the load opposes the external load so that the stresses in the granite shell are kept to a minimum.
Such an arrangement allows the use of a granite roll in an externally loaded application that has not been possible with current granite roll designs.
The deflection compensating pistons can provide a uniform internal pressure along the full length in a cross-machine direction of the shell or such pistons may be segmented to provide variable pressures in either the cross-machine or the machine direction.
In one embodiment of the present invention, the granite sleeve is mounted on a metal support and end rings are used to place the granite sleeve in compression.
Since granite has very little tensile strength, it is normally put into compression so that during operation, the granite will not be subject to any tensile stresses.
Alternatively, according to another embodiment of the present invention, the backing roll includes opposing end plates with spaced holes axially drilled therethrough and a plurality of tie rods extending through corresponding spaced holes disposed axially through the granite shell so that when the tie rods are adjusted to compress the end plates, the granite shell is put into compression and is reinforced by the tie rods.
In either of the aforementioned embodiments, the backing roll may be driven or nondriven.
Therefore, a primary objective of the present invention is to provide a press section apparatus wherein a backing roll includes at least an outer surface of natural granite and deflection compensating means within the backing roll for compensating for any deflection between the shell and the press member.
Another object of the present invention is the provision of a press section apparatus which includes an extended nip press shoe and a backing roll having a granite sleeve and deflection compensating means for compensating for relative deflection between the sleeve and the shoe.
Another object of the present invention is the provision of a press section apparatus including a press member which includes a hydrodynamic shoe which cooperates with a backing roll which includes a shell defining an outer granite surface.
Another object of the present invention is the provision of a press section apparatus which includes a press member having a hydrostatic shoe wherein the backing roll includes a shell which defines a granite outer surface and a deflection compensating means for compensating for any relative deflection between the shell and the press member during passage of the web through the press nip.
Another object of the present invention is the provision of a press section apparatus including an ENP shoe and a backing roll of granite wherein the granite backing roll has deflection compensating means controlled such that during use, pressure exerted by the shoe is counteracted by a deflection compensating means and in the event of any failure in the hydraulic pressure line of the deflection compensating means, the supply of hydraulic pressure to the press shoe is simultaneously released.
Other objects and advantages of the present invention will be apparent to those skilled in the art by consideration of the detailed description and the annexed drawings.